A tire having a visible sidewall of a rubber composition containing a combination of:
(A) a liquid ethylene/butylene copolymer having a terminal hydroxyl group on one end of the copolymer and either a partially hydrogenated polyisoprene block containing olefinic double bonds, or an epoxidized polyisoprene block containing olefinic double bonds, on the other end of the copolymer,
(B) cis 1,4-polybutadiene rubber,
(C) a brominated copolymer of isobutylene and para-methylstyrene, and
(D) a minor amount of at least one conjugated diene-based rubber, preferably cis 1,4-polyisoprene.
In such manner, it is considered herein that such liquid polymers are functional liquid polymers via the associated terminal hydroxyl group on one end of the copolymer and the modified polyisoprene blocks on the other end. In another aspect, the liquid copolymers are seen herein to act as processing aids for the rubber composition followed by becoming an integral part of the polymer network upon sulfur curing the resulting rubber composition. Therefore, a tire having a sulfur cured, visible sidewall rubber composition is also provided. In a further aspect, the rubber composition may be reinforced with a reinforcing filler as carbon black or as a combination of carbon black and silica together with a coupling agent.
Pneumatic tires have sidewalls which are conventionally desired to have good resistance to flex fatigue, scuff resistance and resistance to tear. A suitable visual appearance of the tire sidewall surface may also be a desirable property.
Scuff resistance for a tire sidewall rubber composition is typically enhanced by inclusion of cis 1,4-polybutadiene rubber which is well known to those having skill in such art.
Resistance to flex fatigue for a tire rubber sidewall may sometimes be enhanced by inclusion of an aromatic, naphthenic or parafinnic rubber processing oil in its composition which is also well known by those having skill in such art.
However, while rubber processing oils have been used to improve the processability (e.g. reduced rubber viscosity) of various unvulcanized high viscosity elastomers, as well as improving resistance to flex fatigue of a rubber composition for use as a tire sidewall application, the inclusion of processing oils in such rubber compositions often results in a decrease in various physical properties such as, for example, a vulcanized modulus of elasticity. Accordingly, the use of substantial amounts of processing oils in rubber compounds for a purpose of enhancing their unvulcanized processability, as well as improving resistance to flex fatigue is not necessarily a desirable option.
In one aspect, it has heretofore been proposed to use various diene-based liquid polymers which contain olefinic unsaturation to replace at least a portion of rubber processing oil contained in various rubber compositions. One philosophy has been for the liquid polymer to initially enhance the processability (e.g. reduce rubber viscosity) of a high viscosity unvulcanized rubber composition by reducing its viscosity and to later co-sulfur vulcanize with the elastomer via the olefinic double bonds contained in the liquid polymer upon sulfur vulcanizing the rubber composition.
In another aspect, U.S. Pat. No. 6,255,397 relates to replacement of conventional rubber processing oils with a prescribed hydroxyl terminated liquid polyalkylene-based polymer which reportedly resulted in improved resistance to flex fatigue in the absence of amine-based antioxidants for a sulfur-cured tire sidewall rubber composition which was composed of cis 1,4-polybutadiene, a brominated copolymer of isobutylene and para-methylstyrene and a minor amount of cis 1,4-polyisoprene.
However, it remains desirable to continue to improve various properties for a tire rubber sidewall.
In the description of this invention, the term xe2x80x9cphrxe2x80x9d as used herein, and according to conventional practice, refers to xe2x80x9cparts of a respective material per 100 parts by weight of rubberxe2x80x9d. The terms xe2x80x9crubberxe2x80x9d and xe2x80x9celastomerxe2x80x9d can be used interchangeably, unless otherwise indicated. The terms xe2x80x9crubber compositionxe2x80x9d, xe2x80x9ccompounded rubberxe2x80x9d and xe2x80x9crubber compoundxe2x80x9d can be used interchangeably to refer to xe2x80x9crubber which has been blended or mixed with various ingredients and materialsxe2x80x9d and the terms xe2x80x9ccurexe2x80x9d and xe2x80x9cvulcanizexe2x80x9d may also be used interchangeably herein, unless otherwise indicated and such terms are well known to those having skill in the rubber mixing or rubber compounding art.
In accordance with this invention a tire is provided having a visible sidewall of a rubber composition comprised of, based upon parts by weight per 100 parts by weight rubber (phr):
(A) 100 phr of elastomers comprised of
(1) about 30 to about 70, alternately about 35 to about 65, phr of cis 1,4-polybutadiene rubber,
(2) zero to about 30, alternately about 5 to about 25, phr of cis 1,4-polyisoprene rubber, and
(3) about 30 to about 70, alternately about 40 to about 60, phr of brominated copolymer of isobutylene and p-methylstyrene;
(B) about 2 to about 30, alternately about 5 to about 25, phr of a liquid polyalkylene copolymer selected from:
(1) a first ethylene/butylene copolymer having a mono-terminal hydroxyl group on one end of the copolymer and an hydrogenated polyisoprene block which contains olefinic double bonds therein on the other end of the copolymer, or
(2) a second ethylene/butylene copolymer having a mono-terminal hydroxyl group on one end of the copolymer and an epoxidized polyisoprene block on the other end of the copolymer, and
(C) about 15 to about 70 phr of reinforcing filler as:
(1) carbon black, or
(2) combination of carbon black and synthetic precipitated silica, optionally with a coupling agent having a moiety reactive with hydroxyl groups (e.g. silanol groups) contained on the precipitated silica and another moiety interactive with said conjugated diene-based elastomer(s).
It is a significant aspect of this invention that the rubber composition is suitable for a tire sidewall where high flex endurance properties over a considerable period of time and good resistance to atmospheric ozone degradation without use of amine-based antidegradants in the rubber composition is desired.
In further accordance with this invention, a tire is provided having said visible sidewall rubber composition as a sulfur cured rubber composition. For such tire, the unvulcanized tire assembly, including its sidewall rubber composition, is vulcanized in a suitable mold at an elevated temperature to shape and sulfur-vulcanize the associated rubber compositions of the tire. In such manner, the liquid hydroxyl terminated, saturated ethylene/butylene backbone copolymers having a modified polyisoprene block on the opposite end of the copolymer becomes a part of the sulfur cured network of the sidewall rubber composition in combination with the diene-based rubbers and brominated copolymer of isobutylene and para methylstyrene. Therefore, a tire is provided where said sidewall rubber composition is sulfur vulcanized and said hydroxyl terminated ethylene/butylene backbone copolymer containing said modified polyisoprene block is a part of the sulfur vulcanized rubber network of said sidewall rubber composition.
A significant aspect of this invention is the inclusion of the specified polyalkylene copolymers in a tire sidewall in combination with the cis 1,4-polybutadiene rubber and the brominated copolymer of isobutylene and para-methylstyrene.
Interestingly, it can readily be seen that the liquid copolymers are of a saturated ethylene/butylene backbone containing on one end a mono-hydroxyl terminal group and on the other end of the saturated liquid copolymer backbone is the olefinic (carbon-to-carbon unsaturation) polyisoprene block or the epoxidized polyisoprene block.
In particular, the mono-hydroxyl termination of the copolymers is considered herein to be beneficial in that it might provide compatibility with the diene-based rubbers whereas the ethylene/butylene backbone provides compatibility with the copolymer of isobutylene and para-methylstyrene and the polyisoprene block containing olefinic unsaturation or epoxy groups provides cure compatibility with the cured sulfur network.
And, of course, the use of a liquid copolymer as a processing aid to reduce the rubber viscosity, namely the Mooney (1+4) viscosity, is particularly valuable in order to improve processability of the unvulcanized rubber composition, particularly where relatively high viscosity elastomers are desired, without resorting to utilization of extensive amounts of conventional rubber processing oils.
Thus, a significant aspect of the invention is the reduced use of conventional aromatic, naphthenic or paraffinic rubber processing oils in the sidewall rubber composition.
A significant aspect of the invention is the resultant sidewall rubber composition comprised of such materials which have been observed to exhibit acceptable resistance to flex fatigue property over a considerable period of time as well as acceptable resistance to atmospheric ozone degradation of the rubber composition without requiring an amine-based antioxidant in the rubber composition itself.
In one aspect this is considered herein to be advantageous for a tire sidewall rubber composition in order to improve, or substantially maintain, visible tire sidewall surface appearance after aging while maintaining good durability properties of the sidewall rubber composition itself.
In practice, as hereinbefore pointed out, the sidewall rubber composition may also contain up to about 15, alternately about 2 to about 15, phr of at least one additional diene-based elastomer. Such additional diene-based elastomer may preferably be selected from at least one of organic solvent solution polymerization prepared styrene/butadiene copolymer rubber, aqueous emulsion polymerization prepared styrene/butadiene copolymer rubber, organic solvent polymerization isoprene/butadiene copolymer rubber and styrene/isoprene/butadiene terpolymer rubber. Said additional solvent polymerization prepared rubber may also be a tin coupled elastomer.
In practice, the cis 1,4-polyisoprene rubber for the rubber composition may be natural or synthetic rubber, usually preferably natural rubber.
In practice, it is preferred that the reinforcing filler is carbon black. If desired, silica, particularly precipitated silica which is also intended to include a synthetic precipitated aluminosilicate, (e.g. a synthetic silica precipitated with, or otherwise treated, a very small amount or aluminum) can be present at a level of up to 15 phr (e.g. from about 2 to about 15 phr).
As would be understood by one having skill in such art, a coupling agent would normally be used for said precipitated silica, although the precipitated silica might be used without, and therefore exclusive of, a coupling agent to aid in coupling the precipitated silica to said diene-based elastomers or other ingredients in the rubber composition.
Such coupling agent may be, for example, a bis-(3-triethoxysilylpropyl) polysulfide having an average of from 2 to 2.6 or an average of from 3.5 to 4 connecting sulfur atoms in its polysulfidic bridge. In one aspect of the practice of this invention, such coupling agent is preferably is a bis-(3-triethoxysilylpropyl) polysulfide having an average of from 2 to 2.6 connecting sulfur atoms in its polysulfidic bridge.
The aforesaid first liquid functional copolymer is a liquid polymer having a backbone of an ethylene/butylene copolymer having a terminal mono hydroxyl group on one end of the copolymer and a partially hydrogenated polyisoprene block polymer on the other end of the copolymer. Representative of such functional polymer, a liquid polymer, is Kraton Liquid Polymer L-1302(trademark) from the Shell Chemical Company.
The aforesaid second liquid functional copolymer is a liquid polymer having a backbone of an ethylene/butylene copolymer having a terminal mono hydroxyl group on one end of the copolymer and an epoxidized polyisoprene block polymer on the other end of the copolymer. Representative of such functional polymer, a liquid polymer, is Kraton Liquid Polymer L-207(trademark) from the Shell Chemical Company.
In practice, said liquid functional copolymers may preferably have an equivalent weight range from about 250 to about 70,000, more preferably about 500 to about 7,000, so long as it is liquid at room temperature, (about 23xc2x0 C.), namely that they are readily pourable at such temperature.
The brominated copolymer of isobutylene and para-alkylstyrene for this application is a copolymer comprised of repeat units derived from isobutylene and para-methylstyrene. Preferably, the copolymer is composed of from about 85 to about 99 weight percent units derived from isobutylene.
In practice, it is considered herein that copolymer is post-brominated and has a resultant bromine content of up to about 5 weight percent and, alternately, from about 0.2 to about 1.5 or even up to 2.5 weight percent in the copolymer.
A preferred copolymer is brominated copolymer of isobutylene and para-methylstyrene as, for example, EXXPRO from the Exxon Chemical Company reportedly having a Mooney Viscosity ML(1-8) at 125xc2x0 C. of 50+/xe2x88x9210, an isobutylene content of about 94 to 95 weight percent, and a para-methylstyrene content of about 5 percent, with a total bromine content of about 0.8 weight percent. European patent publication No. EP 0.344.021 contains a description of how to make such copolymer. Also, reference may be made to European Patent Publication No. EP 0801105.
The rubber compositions of this invention can be prepared by simply mixing the liquid polyalkylene polymers into the rubber elastomer. This can be done utilizing a wide variety of mixing techniques. In most cases, the mixing will be carried out utilizing an internal rubber mixer or an open roll (e.g. dual opposing rolls) mill mixer. An internal rubber mixer is preferred and it will generally be preferred to mix the liquid polymers into the elastomer composition during the non-productive compounding stage.
It should be noted that the non-productive compounds do not contain curatives, such as sulfur, or accelerators. On the other hand, productive compounds contain a curative which will cure (vulcanize) the rubber after it is heated to a curing temperature.
The rubber compositions of this invention will frequently contain a variety of additional compounding ingredients and/or additives. Typical amounts of processing aids and rubber compounding ingredients comprise about 1 to about 50 phr. Such processing aids can include, for example, aromatic, naphthenic, and/or paraffinic processing oils. Stearic acid is typically referred to as a xe2x80x9crubber compounding ingredientxe2x80x9d. As purchased, it typically contains primarily stearic acid with small amounts of at least one of oleic acid, linolenic acid and/or palmitic acid. The mixture may also contain small amounts (less than about six weight percent) of myristic acid, arachidic acid and/or arachidonic acid. Such material, or mixture, is conventionally referred to in the rubber compounding art as xe2x80x9cstearic acidxe2x80x9d.
The vulcanization is conducted in the presence of a sulfur-vulcanizing agent. Examples of suitable sulfur-vulcanizing agents include elemental sulfur (free sulfur) or sulfur donating vulcanizing agents, for example, an amine disulfide, polymeric polysulfide, alkyl phenol polysulfides or sulfur olefin adducts. As known to those skilled in the art, sulfur-vulcanizing agents are used in an amount ranging from about 0.5 to about 4 phr, or even, in some circumstances, up to about 8 phr, with a range of from about 1.5 to about 2.5, sometimes from 2 to 2.5, being preferred. It is to be appreciated, however, that the cure system, including desired cure package ingredients as well as the following discussion concerning cure accelerators, may vary depending upon the rubber compound ingredients, including the chosen polymers and elastomers.
Accelerators are used to control the time and/or temperature required for vulcanization and to improve the properties of the vulcanizate. In one embodiment, a single accelerator system may be used, i.e., primary accelerator. Conventionally and preferably, a primary accelerator(s) is used in total amounts ranging from about 0.5 to about 4, preferably about 0.8 to about 2.8, phr. In another embodiment, combinations of a primary and a secondary accelerator might be used with the secondary accelerator being used in smaller amounts (of about 0.05 to about 3 phr) in order to activate and to improve the properties of the vulcanizate. Combinations of these accelerators might be expected to produce a synergistic effect on the final properties and are somewhat better than those produced by use of either accelerator alone. In addition, delayed action accelerators may be used which are not affected by normal processing temperatures but produce a satisfactory cure at ordinary vulcanization temperatures. Vulcanization retarders might also be used. Suitable types of accelerators that may be used in the present invention are amines, disulfides, guanidines, thioureas, thiazoles, thiurams, sulfenamides, dithiocarbamates and xanthates. Some representative examples of primary accelerators which can be utilized include thiazole accelerators, such as benzothiazyldisulfide and 2-mercaptobenzothiazole; sulfenamide accelerators, such as N-oxydiethylene benzothiazole-2-sulfenamide, N-t-butyl-2-benzothiazolesulfenamide and N-cyclohexyl-2-benzothiazolesulfenamide; dithiocarbamate accelerators, such as bismuth dimethyldithiocarbamate, cadmium diethyldithiocarbamate, copper dimethyldithiocarbamate, lead dimethyldithiocarbamate, selenium diethyldithiocarbamate, tellurium diethyldithiocarbamate and zinc dimethyldithiocarbamate; thiuram accelerators such as dipentamethylene thiuram hexasulfide, tetramethylthiuram monosulfide and tetraethylthiuram monosulfide; and thiourea accelerators, such as trimethyl thiourea and dimethylethyl thiourea. If a second accelerator is used, the secondary accelerator is preferably a guanidine, dithiocarbamate or thiuram compound.
As hereinbefore discussed in an alternative embodiment of this invention, the formulation can additionally contain silica, usually with a silica coupling agent; wherein the amount of particulate silica, namely an amorphous silica, ranges from about zero to about 15, optionally about 2 to about 10 or 15 phr and, wherein the silica coupling agent, if used, is used in an amount wherein the weight ratio of silica coupler to silica may be from about 0.1/1 to about 0.2/1.
The commonly employed siliceous pigments used in rubber compounding applications can be used as the silica in this invention, including pyrogenic and precipitated siliceous pigments (silica), although precipitated silicas are preferred.
The siliceous pigments preferably employed in this invention are precipitated silicas such as, for example, those obtained by the acidification of a soluble silicate, e.g., sodium silicate.
The following examples are used to illustrate the invention. The parts and percentages are by weight unless otherwise indicated.